The basic meaning doesn't change with the substitutions for A and B, however there is a change in tone or mood. "Listen in" - in particular with the original sentence A - implies an intimacy and enthusiasm on the part of the listeners that you lose completely if you omit the "in."
With broadcast programs the 'in' is taken, I think, from the verb 'to tune in'. Tuning in to a particular station need not be at the beginning of the program, so while "listen" gives the impression of listening to an entire message, "listen in" gives the impression of listening to any or all of the message as is convenient to the listener. Listening in is like overhearing. The speaker need not